You use LUA viewer to write scripts. I add new scripts by editing the Ultimate Free Ride Mod. Working with LUA viewer is pain in the ass though. If you want to know more, you should join our Discord server and go into the community chat and type "script", "LUA" etc. in the search bar.

Textures in Mafia 2 are saved as DDS, they are located in SDS packs, you unpack them using SDS GUI.

Placing objects, actors and editing lighting is not possible at the moment, as far as I know. You can only replace cars and place cars around the map with scripts.

As T3mas1 said, there currently isn't any way to modify the map in Mafia 2, but there are modders working on ways to build tools to allow them to edit maps in Mafia 2. Modders have already figured out ways to modify vehicle models and put them in the game, and some have even found workaround solutions for editing or adding in map objects, such as in the thread that T3mas1 posted. I would also highly recommend checking out the thread I post, as it highlights a lot of the new modding developments for Mafia 2. It's a very interesting read if you care to check it out. You can find it on these forums at this link:
http://mafiascene.com/forum/viewtopic.php?t=10315&postdays=0&postorder=asc&start=0